Regards the Mag idler:  I had to deal with this problem when I installed dual LSE systems on my TSIO-550 powered LIV.  Ron Monson was building up the motor and suggested making a custom part to replace the mag gear if I wanted to install a second alternator on the aft side of the pad--or I could install the bottom half of an old mag as a "place holder".  I was aware of a few incidences of stray accessory idlers when mags were removed without removing or securing the idler gear, mostly on Lycs, causing premature engine overhaul so this was a challenge requiring some thought.  Since mine was one of the first dual LSE installs on 6 cyl Continentals (plasma IIIs which use a crank sensor) we were breaking some new ground.  Ron has done many in the several years since so you might give him a call to see what he would recommend.  I just removed both mags and all mag accessory gears during engine buildup and did not install a second alt so there are many fewer teeth to make metal if something goes astray.  Two Odyssey batteries and a certified alt seemed sufficient and have worked well for the past 140 hours.

I know parallel busses each with an alt and battery for electrically dependent engines are the default design, but I think three power providers each separable from the main buss eliminate some complexity and offer great redundancy, especially when coupled with a good over voltage protector, current limiter and load meter.  For an even simpler power distribution system you could follow the LSE dual installation manual and use a small back up battery with a Schottky Diode to eliminate the extra alt and solve the idler gear problem.  Choices, choices--your decisions may differ.

regards, paul--N94PT